This invention relates to improvements in a welded structure in which a welded part such as fuel tube or the like and a fuel filling container such as a fuel tank or the like are connected with each other under welding, and in a method of welding the welded part and the fuel filling container to form the welded structure.
Hitherto a fuel filling container such as a fuel tank or the like has been in general formed of a material whose main component is high density polyethylene (HDPE) under blow molding from the viewpoints of moldability, fuel resistance, impact resistance and production cost. Additionally, a multi-layer (for example, 5 layers) structure has been recently extensively employed for the wall of the fuel filling container, in which a layer of high density polyethylene is bonded through an adhesive layer to a barrier layer of ethylene-vinyl alcohol copolymer or nylon resin.
Concerning the welded part to be installed to the fuel filling container, it is general to form the welded part of a synthetic resin which is the same with or similar to that of the fuel filling container because the welded part formed of such a resin can be bonded to the fuel filling container by a simple welding operation using a heated plate or the like. In this regard, the welded part formed of high density polyethylene, modified polyolefin or the like is extensively used. However, it is essential to reduce fuel permeation through the welded part in order to cope with future fuel permeation regulation. Hence, it is eagerly required to so arrange the welded part as to be reduced in fuel permeation as compared with the welded part formed of high density polyethylene. An example of similar techniques is disclosed in U.S. Pat. No. 5,139,043.